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Carbon Nanotube Reinforced Aluminium Matrix

Composites – A Review

Shadakshari R1, Dr.Mahesha K

2, Dr.Niranjan H B

3

1Asst Professor, Department of Mechanical Engineering, Acharya Institute of Technology, Bangalore, Karnataka, India.

2Professor, Department of Mechanical Engineering, Acharya Institute of Technology, Bangalore, Karnataka, India.

3Director of Research and PG Studies, HKBK College of Engineering, Bangalore, Karnataka, India.

Abstract: The present paper illustrates mixing procedures for Al-CNTs powder preparation alongside depiction of the CNTs

dispersion results from the different mixing techniques. XRD analysis showed that the mean grain size of powders milled for 6

h was found to be 48.4 nm and for extrudates of CNT-Al, it is 56.6 nm. Based on the geometry and physical properties of

multiwalled nanotubes, three strengthening mechanisms were considered for CNT/Al composite system. It was evident from the

testing that as the content of nanotubes in the matrix increased, the micro-hardness measured on the Vickers scale also

increased. The investigation of the damping behaviour of 2024Al-CNT composite showed that the damping capacity of the

composite with a frequency of 0.5 Hz reaches 975 x 10-3, and the storage modulus is 82.3 GPa when the temperature is 400˚ C,

which shows that CNTs are a promising reinforcement for metal matrix composites to obtain high damping capabilities at an

elevated temperature without sacrificing the mechanical strength and stiffness of a metal matrix.

Key words: Composites, differential scanning calorimetry (DSC), microstructure. Carbon nanotube

I.INTRODUCTION

Single and multi-wall carbon nanotubes have created tremendous expectations as strengthening additives for metallic, ceramic

and polymer composites due to their high strength and stiffness. Metal matrix/CNT composites (MM/CNT) have a great

potential in load bearing applications and electronic packaging due to their high specific strength, high thermal conductivity and

low coefficient of thermal expansion. These properties are advantageous in advanced applications like aerospace and

automotive structural members where a lower weight leads to savings in energy. In recent years, much research has been

focused on the development of CNT reinforced Al matrix composites, because Al matrix composites have been wide prospects

of applications in aviation, spaceflight and automobile industries[1]. CNTshave a Young’s modulus of 1TPa, making them ideal

reinforcements for composite materials. It is important to understand the relevant strengthening mechanisms involved in

CNT/Al composites, in order to produce optimized composites [1]. The carbon nanotubes having excellent chemical stability

due to their seamless cylindrical graphite structure are an exceptional candidate for the reinforcement in aluminum

matrix[2].The quality of dispersion, however, is a crucial factor which determines the homogeneity and final mechanical

properties of these composites[3].

II. Al-CNTs Powder Preparation

0.5% of CNTs was added to the Al powders (Al-0.5 wt% CNT) and three different mixing methods, namely high energy and

low energy ball millings, and polyester binder-assisting (PBA) technique were used[3,4,5,6,].

A. High energy ball milling

Al-0.5 wt% CNT powders were mixed by stirring before being transferred into the ball milled jar. Mechanical alloying was

performed on a Retch PM400, Germany planetary ball mill machine. To prevent severe cold rolling, stearic acid (2 wt %) was

added to the mixture during the ball milling. Ball (agate type) to powder weight ratio was 5:1 with a revolution speed of 200

RPM. Ball milling time was 4 h, with an interim period of 30 min for every one hour, in order to prevent over-heating.

B. Low energy ball milling

The same amount of Al-CNTs powders and balls as used in high energy ball milling were loaded into a plastic container then

blended by a horizontal rolling machine. The mixture was rolled for 4 h continuously with a speed of 200 rpm.

C. Polyester binder-assisted (PBA) mixing

A polyester binder assisted (PBA) method for coating CNTs on Al powder was used. Firstly, polyethylene glycol (PEG) with a

molecular weight of 20,000 (MW. 20,000) in a form of flake was hand mixed with CNTs in a mortar. The mixture was

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transferred into a Haake twin screw mixer. To effectively disperse the CNTs within the PEG, the mixture was melt blended for

20 min, at the desired melting point of PEG, i.e. 70˚ C, using a speed of 60 rpm. Subsequently the Al powder was fed into the

viscous PEG-CNTs mixture. The compound was continuously blended for another 30 min with the same rotational speed to

obtain a good dispersion of CNTs in the mixture. The volume fraction of PEG to Al was 4:6. The Al-CNTs-PEG precursor was

cooled cured and heated at 400˚ C in an argon atmosphere for 3 h to thoroughly decompose the PEG. The schematic of the

processing procedure is presented in Fig.1a and the schematic of the twin screwers is shown in Fig1b.

Fig.1a Illustration of the mixing procedures of Al-CNTs powders by polyester binder-assisted (PBA) technique; and the (b) schematic of the twin-screw mixer

[3]

The summary of the CNTs dispersion results from the different mixing techniques is schematically presented in fig 2 and Table

1[3]. TABLE 1

THE SUMMARY OF THE CNTS DISPERSION RESULTS FROM THE DIFFERENT MIXING TECHNIQUES [3]

Mixing technique Al powder

morphology

C N T morphology Dispersion effect

P B A Round shape Negligible charge Coated with Al powder through

agglomerates still existed

Low energy ball milling Round shape with

minor change

Medium change Dispersed within Al powder though

agglomerates still existed

High energy ball milling Round shape with

severe change

Severe damage Effectively dispersed though insufficiently

distributed

Fig2 Schematic depictions of CNTs and Al powder after different mixing techniques. In (a) high energy ball milling, CNTs were effectively dispersed though

insufficientlydistributed within the Al powders; and in (b) low energy ball milling, CNTs dispersed within Al powder though agglomerates still existed; and in (c) the PBA method, CNTswere coated with Al powders though agglomerates still existed. [3]

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Ball milled samples containing CNTs exhibited high notch sensitivity and consistently fractured outside the gauge length, which

was not the case for pure milled aluminium samples. Fig.3 shows nanoindentation and Vickers hardness results for extruded

samples of un-milled and milled Al, in addition to the milled Al-2 wt% CNT composites. The hardness is seen to increase

significantly (approx. 3 times) for extrusions of ball milled powders, due to excessive strain hardening. XRD analysis showed

that the mean grain size of powders milled for 6 h was found to be 48.4 nm and for extrudates of CNT-Al, it is 56.6 nm which is

only slightly greater than the grain size of ball milled powders, thus suggesting that extrusion at 500˚ C retains the

nanostuctutre[5].

Fig 3 Vickers micro-indentation and nano-indentation hardness of un-milled and milled pure Al and Al–2 wt% CNT extruded samples [5]

III.Fabrication Techniques

A. Powder Metallurgy

The aluminum powder (purity of 99.0%, grain size of 70µm) and CNTs were homogeneously mixed by hand grinding for

30min, and ethanol was added to avoid possible oxidation. The mixture of CNTs and aluminum powder was uniaxially pressed

in steel dies under a pressure of 300MPa for 2 min. The specimens were isothermally sintered at 100˚C for 1 h and 600˚C for 2

h in a pure argon atmosphere. The specimens for testing, containing 0 to 3% of CNTs respectively, were 60 mm X 10 mm X 2

mm in size [1].

Multiwalled carbon nanotubes (MWNTs) were treated with the reflux within the concentrated nitric acid for 0-25 h to purify

and disperse the tangled MWNTs. The results show that the reflux time markedly affects the morphology of MWNTs [7].the

weight loss of MWNTs increase as the reflux time increases. Meanwhile, the dispersion of MWNTs with 0-2.0 wt. % in 2024Al

powders using mechanical stirring with an assisting ultrasonic shaker in ethanol was also studied. When the content of MWNTs

is less than 1.0 wt. %, MWNTs can uniformly distribute on the surface of 2024Al powders; however, when the content of

MWNTs is 2.0 wt. %, MWNTs entangled with each other on the surface of 2024Al powders[7].

B. Compocasting

A356 aluminium alloys reinforced with CNTs were produced by stir casting and Compocasting routes. In order to alleviate the

problems associated with poor wettability, agglomeration and gravity segregation of CNTs in the melt, CNTs were introduced

into the melts by injection of CNT deposited aluminum particles instead of raw CNTs. Aluminum particles with a mean

diameter of less than 100µm were first deposited by CNTs using Ni-P electro less plating technique and then injected into the

melt agitated by a mechanical stirrer. The slurry was subsequently cast at a temperature corresponding to full liquid as 0.15 and

0.30 solid fractions

Fig4 SEM images of aluminum particles: (a) Before coating; (b) After coating; (c) Higher magnification of the area marked byrectangle in (b) [8]

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Fig.4 Illustrates the SEM images of aluminum particles before and after coating. It is shown that a proper mechanical

interaction between aluminum particles and Ni-P-CNT composite coating has been achieved. It is also evident that the coating

has increased the average size of the aluminum particles. The higher magnification image of the area marked by the rectangle

infig4bis shown in fig 4c reveals the characteristic morphology of the Ni-P electroless coating on the aluminum particle and,

more importantly, a very uniform distribution of CNTs in the Ni-P-CNT composite coating. Due to gradual co-deposition of Ni-

P and CNTs on the aluminium particles, no CNT agglomerates can be observed in the composite coating. The results show that

the addition of CNTs to A356 matrix can significantly refine both full liquid and semi-solid cast microstructures. Hardness of

the samples is also significantly increased by the addition of CNTs and A356-CNT composite cast at 0.3 solid fractions

produces highest hardness [8].

C. High Pressure Torsion (HPT)

Fig.5 shows severe plastic deformation by high pressure torsion (HPT) of powders at elevated temperature (473 K) was

employed to achieve both powder consolidation and grain refinement of aluminum matrix nanocomposites reinforced with 5

Vol% CNTs. Before the HPT, the powders were ball milled using a planetary ball mill in order to achieve a molecular level

mixing. Aluminum was treated by the same process for a reference. The HPT processed disk was composed of considerably

equilibrium grain boundaries with high misorientaiton angles. The CNT–reinforced ultrafine grained microstructural features

resulted in high strength and ductility [9].

Fig 5 Schematic illustration of HPT facility and operation with a pressure showing the first compressive stage and the next torsional stage [9]

IV. Chemical stability of CNTs in Al matrix

For CNTs / Al composites, an important issue of the chemical stability of the CNTs in the Al matrix, is whether or not CNTs

can be applied to the Al matrix as reinforcement. The DSC pattern of the mixture of CNTs and 2024 Al powders is shown in

Fig.6. It was distinctly seen that a sharp exothermic peak appeared beyond the Al melting peak, which could be Al4C3 carbides

formed in the reactions of CNTs – Al [10, 11].

Fig6 DSC of a mixture of CNTs and 2024Al alloy powders. [10]

V. Strengthening Mechanisms

Based on the geometry and physical properties of multiwalled nanotubes, three strengthening mechanisms were considered for

CNT/Al composite system, namely thermal mismatch, Orowan looping and shear lag models.

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A. Thermal mismatch

There exists a significant coefficient of thermal expansion mismatch between the matrix and the carbon nanotubes, which

would result in the prismatic punching of dislocations at the interface leading to work hardening of the matrix. The dislocation

density generation is likely to be higher, which in turn would result in increased strengthening. Incremental strength, ∆σ = αµρ

(1/2) b Where α = constant = 1.25, µ is the modulus of rigidity of the matrix (Al), b is Burger’s vector. Dislocation density (ρ) is

given by ρ = 10Aε/(bt(1-A)), where A is the reinforcement (carbon nanotube) volume fraction, ε is thermal strain, b is Burgers

vector, and t is the dimensions of the reinforcement.

B. Orowan looping

In this mechanism the motion of the dislocation is inhibited by nanometer sized carbon nano tubes, leading to bending of these

dislocations between the carbon nanotubes. This produces a back stress, which will prevent further dislocation migration and

results in an increase in yield stress. The Orowan looping mechanism is important in aluminum alloys, which are strengthened

by fine precipitates. Since carbon nanotubes effectively represent very fine particles, perpendicular to the tube axis, in the order

of a few nanometers, it strengthens the aluminum matrix. Incremental shear strength of the composite ∆τ=Constant * µbA(1/2)

/ r

* ln(2r/ro) where constant = 0.093 for edge dislocation and 0.014 for screw dislocation, ro is the core radius of dislocation = 3.5

x 10-9

m, b is the Burger’s vector, µ is the modulus of rigidity of matrix (Al) = 2.64 x 101-9

N/m2, and r is the volume

equivalent radius = 1.593 x 10-7

m for MWNT and 7.087 x 10-9

m for SWNT.

C. Shear lag

This model involves the transfer of load from the matrix to the reinforcement by interfacial shear stress. Thus the stiffness of the

carbon nanotubes is directly utilized. The Young’s modulus (Ec) of the composite is Ec = A Ef (1-(tan h ( ns))/ (ns)) + ( 1-A ) Em

where Ef is the Ypung’s modulus of the reinforcement (CNT), n = (2 Em / ( Ef ( 1+γm ) ln ( 1/ A )))(1/2)

, Em is the Young’s

modulus of matrix (Al), s is the aspect ratio of the reinforcement (CNT), A reinforcement (CNT) volume fraction , γm =

Poisson’s ratio of Al matrix, and aspect ratio s = 100 for MWNT and 500 for SWNT. The strengthening of the composite could

be due to the synergetic effect of these mechanisms[12].

VI. Properties

The effects of CNT content on the relative density, the hardness, and the friction and wear behaviour of the CNT/Al composites

under dry sliding conditions were investigated [1].

A. Micro-hardness

The investigations have been extended to carry out the micro-hardness measurements on composite samples containing 1, 2, 4

and 10 wt% of carbon nanotubes reinforced aluminium matrix. It was evident from the testing that as the content of nanotubes

in the matrix increased, the micro-hardness measured on the Vickers scale also increased. Fig.7 Illustrates the different values of

micro-hardness with the quantity of nanotubes reinforced in the matrix [2].

Fig.7 Change in micro hardness and yield strength with respect to increase in content of the CNT in aluminum matrix [2]

B. Electrical properties

There is a slight increase in electrical resistivity with the addition of carbon nanotubes. The carbon nanotubes have, in any case,

a lower electrical conductivity than aluminium. In addition, if the carbonnanotubes are agglomerated at the Al grain boundaries,

they can form a kind of grain boundary phase which will increase the scattering of the charge carrier at grain boundaries, hence

reducing the conductivity. The carbide phases, which are also poorly conducting, and the porosity, which is larger than pure Al,

can also influence the electrical resistivity of the composites.

From room temperature on, the electrical resistivities decreased linearly as the temperature decreased, which is typical metallic

behaviour. At about 80 K there is an abrupt drop (>90%) in the electrical resistivity of the Al-carbon nanotube composites.

Below this transition temperature the resistivity remains at the level of “zero resistance” down to the helium temperature. Such

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a phenomenon resembles a superconducting transition. There has been no explanation for these observations [13].The tensile

yield and ultimate strength of Al-MWCNT increased by 90% with 2 wt% addition of MWCNTs. [4]

C. Wear

The results indicate that the addition to the aluminium matrix of 2.0% (mass fraction) carbon nanotube causes the increase in

the Vickers hardness of about 80%. Within the range of carbon nanotubes content from 1.0% to 2.0% both the friction co-

efficient and wear rate composites decrease with the increase of carbon nanotube content. The delamination wear is the main

wear mechanism for the composites [1].

D. Damping Characteristics

The damping behaviour of 2024Al-CNT composite was investigated with a frequency of 0.5, 1.0, 5.0, 10, 30 Hz at a

temperature of 25-400˚C. The experimental results show that the frequency significantly affects the damping capacity of the

composite when the temperature is above 230˚ C; meanwhile, the damping capacity of the composite with a frequency of 0.5

Hz reaches 975 x 10-3, and the storage modulus is 82.3 GPa when the temperature is 400˚ C, which shows that CNTs are a

promising reinforcement for metal matrix composites to obtain high damping capabilities at an elevated temperature without

sacrificing the mechanical strength and stiffness of a metal matrix. There are three possible mechanisms that could be

responsible for the observed increase in mechanical damping: (1) high inherent damping of carbon nanotubes with graphite

structure, (2) energy dissipation caused by interfacial sliding at the nanotube-matrix interface at an elevated temperature and (3)

energy dissipation caused by a small quantity of micro voids that exists in the composites fabricated by the powder metallurgy

technique [14].

E. Toughness of aluminium–carbon nanotubecomposites

Fig.8 shows the effect of CNT addition on the ratio of the failure strain of Al/CNT composite to that of un-reinforced Alsample

prepared by the same route. It is observed that except one study [4]. all studies observe a reduction in the ductility of the

composite due to CNT addition. There is a large scatter in the data for similar compositions showing that ductility is sensitive to

processing. Two trends can be observed from fig 8the first trend is obtained from the tensile data on the samples prepared by

Deng et al. and Esawi and Borady[15].Which indicates that poor dispersion and poor CNT–matrix bonding leads to low

strengthening as well as a decrease in the ductility. A poor CNT–matrix bonding can be susceptible to cracking nucleation

under tension. Due to the small size of the CNTs, the cracks may be smaller and below the critical flaw size to create any

damage. However overlapping CNTs could lead to interconnected cracks which will cause premature failure of the composite.

The second trend is obtained from data from Choi et al. And He et al[15]. which show that strengthening is achieved at the

expense of ductility. This is a commonly observed observation in fibre reinforced composites resulting due to the smaller

ductility of the fibre. So there is a trade-off that needs to be made between strength and ductility.

Fig 8 Effect of CNT content on the ratio of the failure strain of Al/CNT composite (εf, C) to un-reinforced Al sample (εf, M)[15]

Fig.9 shows the percentage change in the toughness of Al/CNT composites over Al sample prepared by the same route. The

measure of toughness is obtained by the product σUTSεf. It is observed that there is a lot of scatter in the data. A decrease in the

toughness is observed when CNT dispersion is poor as shown by the data by Deng et al. and Esawi and Borady[15].At high

CNT concentrations above 5 vol.% CNTs, there is a decrease in the toughness of the composite. The effect of the increase in the

strength due to good dispersion is suppressed by the decrease in ductility, even for good dispersion.

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Fig.9 Effect of CNT content on change in the toughness of Al/CNT composites over Al sample[15]

VII.CONCLUSION

Ball milled samples containing CNTs exhibited high notch sensitivity and consistently fractured outside the gauge length. It is

shown that a proper mechanical interaction between aluminum particles and Ni-P-CNT composite coating has been achieved. It

is also evident that the coating has increased the average size of the aluminum particles. The HPT processed disk was composed

of considerably equilibrium grain boundaries with high misorientaiton angles. The CNT–reinforced ultrafine grained

microstructural features resulted in high strength and ductility. The strengthening of the composite could be due to the

synergistic effect of mechanisms thermal mismatch, Orowan looping and shear lag models. CNTs are a promising

reinforcement for metal matrix composites to obtain high damping capabilities at an elevated temperature without sacrificing

the mechanical strength and stiffness.

ACKNOWLEDGMENT

We would like to acknowledge the Principal and the Management of Acharya Institute of Technology, for having supported to

carry out research activity. As we have incorporated few sketches/graphs in our paper which are taken from the papers

published in Science Direct, Springer, Pergamon, Materials Research Society and Science Press. We wish to acknowledge the

publishers too.

REFERENCES

[1]. JIANG Jin-long, WANG Hai-zhong YANG hua ,XU Jin-cheng Fabrication and wear behavior of CNT/Al composites Trans.Nonferrous Met.

Soc.China 17(2007 pp113-116 [2]. A.K. Srivastava, C. L. Xu, B. Q. Wei, R. Kishore & K. N. Sood (2008)“Microstructural features and mechanical properties of carbon nanotubes

reinforced aluminum-based metal matrix composites”. Indi. Jour. of Engg. & Mate. Scie. Vol.15, (2008) pp. 247-255. [3]. JinzhiLio, Ming-Jen Tan, (2011), “Mixing of carbon nanotubes (CNTs) and aluminum powder for powder metallurgy use, Powder Technology 208,

(2011), PP42-48

[4]. A.Esawi a, K. Morsi b Elsevier Dispersion of carbon nanotubes (CNTs) in aluminum powder Composites PartA38(2007) P 646-650 [5]. A.M.K.Esawi, K.Morsi,A.Syed, A.AbdelGawad, P.Borah, (2009) “Fabrication and properties of dispersed carbon nanotubes-aluminium composites”,

Material Science and Engineering A 508, (2009) PP167-173.

[6]. Choi H, Shin J, Min B, Park J, Bae D. Reinforcing effects of carbon nanotubes in structural aluminum matrix nanocomposites. J Mater Res 2009;24:2610–6.

[7]. DENG Chunfenga, ZHANG Pengb, ZHANG Xuexic, and WANG Dezunc Dispersion of multiwalled carbon nano tubes in aluminum powders Rare

MetalsVol 28.No2 2009.P.175-180 [8]. B. Abbasipour, B. Niroumand, S. M. MONIR VAGHEFI “ Compocasting of A356-CNT composite” Trans.Nonferrous Met. Soc.China 20 (2010) pp

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[9]. Soo-Hyun Joo, SeungChae Yoon, Chong Soo Lee, Dong Hoon Nam, Soon Hyung Hong, HyongSeop Kim “Microstructure and tensile behavior of Al and Al-matrix carbon nanotube composites processed by high pressure torsion of the powders” J Mater Sci (2010) 45:4652–4658

[10]. Chunfeng Deng , XueXi Zhang, Dezun Wang, Qiang Lin, Aibin Li Preparation and characterization of carbon nanotubes/aluminum matrix

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[12]. R. George, K.T. Kashyap, R. Rahul, S. Yamdagni “Strengthening in Carbon Nanotube/ Aluminium (CNT/Al) Composites. ScriptaMaterialia,Vol. 53, (2005) pp. 1159-1163.

[13]. C. L. Xu, B. Q. Wei, R. Z. Ma, J. Liang, X. K. Ma and D. H. Wu “Fabrication of aluminum-carbon nanotube composites and their electrical

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Biography

R.Shadakshari, BE, M.Tech, presently working as an Assistant Professor in the Department of Mechanical Engineering of

Acharya Institute of Technology, Bangalore. He is pursuing Ph.D in the field of nano-composites from Viswesvaraya

Technological University, Karnataka, India. He holds Life Time Membership of Indian Society for Technical Education

(LMISTE) and Material Research Society of India (MRSI). He has presented a Paper in International conference and published

a paper in an International Journal. His key research area of interest is Nanocomposites.

Dr.Mahesha.K, BE,ME,Ph.D, presently working as a Professor and Head of the Department of Mechanical Engineering,

Acharya Institute of Technology. He holds Life Time Membership of Indian Society for Technical Education (LMISTE) and

Material Research Society of India (MRSI). He has published several papers in International Journals. He is presently guiding 4

Ph.D research scholars. His research area is in advanced materials and Damping characteristics of materials.

Dr.H.B.Niranjan, BE,ME,PhD, presently working as Director of Research and PG studies, in HKBK college of Engineering,

Bangalore. His specialisation is Composite Materials. He holds a Life Time Membership of Indian Society for Technical

Education and Material research society of India. He has published several papers in International Journals. He has guided

several research scholars and is presently guiding 3 Ph.D research scholars.